The present invention relates to a method and apparatus for determining the state of a battery. More specifically, the present invention relates to a method for improving estimation of battery states including state of charge, internal impedance and state of health.
In today""s automotive market, there exists a variety of propulsion or drive technologies used to power vehicles. The technologies include internal combustion engines (ICEs), electric drive systems utilizing batteries and/or fuel cells as an energy source, and hybrid systems utilizing a combination of internal combustion engines and electric drive systems. The propulsion systems each have specific technological, financial, and performance advantages and disadvantages, depending on the state of energy prices, energy infrastructure developments, environmental laws, and government incentives.
The increasing demand to improve fuel economy and reduce emissions in present vehicles has led to the development of advanced hybrid vehicles. Hybrid vehicles are classified as vehicles having at least two separate power sources, typically an internal combustion engine and an electric traction motor. Hybrid vehicles, as compared to standard vehicles driven by an ICE, have improved fuel economy and reduced emissions. During varying driving conditions, hybrid vehicles will alternate between separate power sources, depending on the most efficient manner of operation of each power source. For example, a hybrid vehicle equipped with an ICE and an electric motor will shut down the ICE during a stopped or idle condition, allowing the electric motor to propel the vehicle and eventually restart the ICE, improving fuel economy for the hybrid vehicle.
Hybrid vehicles are broadly classified into series or parallel drivetrains, depending upon the configuration of the drivetrains. In a series drivetrain utilizing an ICE and an electric traction motor, only the electric motor drives the wheels of a vehicle. The ICE converts a fuel source to mechanical energy to turn a generator that converts the mechanical energy to electrical energy to drive the electric motor. In a parallel hybrid drivetrain system, two power sources such as an ICE and an electric traction motor operate in parallel to propel a vehicle. Generally, a hybrid vehicle having a parallel drivetrain combines the power and range advantages of a conventional ICE with the efficiency and electrical regeneration capability of an electric motor to increase fuel economy and lower emissions, as compared with a traditional ICE vehicle.
Battery packs having secondary/rechargeable batteries are an important component of hybrid or electrical vehicle systems, as they enable an electric motor/generator (MoGen) to store braking energy in the battery pack during regeneration and charging by the ICE. The MoGen utilizes the stored energy in the battery pack to propel or drive the vehicle when the ICE is not operating. During operation, the ICE will be shut on and off intermittently, according to driving conditions, causing the battery pack to be constantly charged and discharged by the MoGen. The state of charge (SOC, defined as the percentage of the full capacity of a battery that is still available for further discharge) is used to regulate the charging and discharging of the battery.
Rechargeable batteries are also an important component in other applications where a battery pack is continually cycled, such as in solar-powered battery packs for satellites, portable communication apparatus, laptop computers and wireless transceivers such as those used in radios, cell phones, pagers, etc.
The preferred embodiment of the present invention utilizes a nickel metal hydride (NiMH) battery in the battery pack. A NiMH battery stores hydrogen in a metal alloy. When a NiMH cell is charged, hydrogen generated by the cell electrolyte is stored in the metal alloy (M) in the negative electrode. Meanwhile, at the positive electrode, which typically consists of nickel hydroxide loaded in a nickel foam substrate, a hydrogen ion is ejected and the nickel is oxidized to a higher valency. On discharge, the reactions reverse. The reaction at the negative electrode is more clearly shown by the following reaction diagram:
MHx+OHxe2x88x92←xe2x86x92MHxxe2x88x921+H20+exe2x88x92
The discharging direction is represented by xe2x86x92. The charging direction is represented by ←.
On discharge, OHxe2x80x94 ions are consumed at the negative hydride electrode and generated at the nickel oxide positive electrode. The converse is true for the water molecules.
A difficulty with NiMH batteries is predicting their SOC because of the charging and discharging characteristics of NiMH battery technology. Referring to FIG. 1, typical charge increasing 10 and charge decreasing 12 curves are illustrated for a NiMH battery. Referencing points A and B and points C and D, it can be shown that the voltages are the same while the SOCs are substantially different. Thus, it is very difficult to use an open circuit voltage to accurately predict the SOC of the NiMH battery, as the battery operating operation (charge increasing, charge sustaining or charge decreasing) must be known. Furthermore, coulombic integration methods to determine the SOC of a battery suffer from accumulated errors. When used with a hybrid vehicle, the intermittent charging and discharging of the battery pack amplifies the problems associated with predicting the SOC of a NiMH battery back. To successfully operate a hybrid powertrain of a vehicle incorporating a battery pack, an accurate and repeatable estimate of battery SOC is needed.
Furthermore, charging on hybrid vehicle battery systems has traditionally been fairly static and controlled to a fixed setpoint. The constant power/current in hybrid vehicle usage makes it difficult to estimate battery impedance.
The present invention includes a method to determine the state of a battery, a battery, and an apparatus that can be controlled to affect either the current or voltage at the terminals of the battery. The method to estimate the state may estimate the state of charge, state of health and power capability of the battery. The battery may be a single cell, a battery of cells or a pack of batteries. The preferred battery utilizes NiMH chemistries. However, any other battery technology known in the art such as lead acid, lithium polymer, etc., can be used. The apparatus may consist of a power source that can be varied, or a load that can be varied in a controlled manner. Examples of power sources include generators in a conventional vehicle, the generators in a hybrid vehicle, the charger for an EV, and the charger in consumer electronics. Examples of loads include DC/DC converters in a vehicle, an electrically driven compressor in a vehicle, and the processor in a laptop computer.
The method of the present invention includes introducing a wide spectrum signal (i.e., white noise, white light) into the charging set point of a battery, causing the current to constantly change at the terminals of the battery. The resulting signal is then passed to the charge control logic of the electrical control system utilized to propel a vehicle. The resulting signal provides the control logic with an improved ability to identify the impedance, open circuit voltage, SOC, and power limits of the battery pack.
The present invention further includes a vehicle having both parallel and series hybrid drive systems incorporating a hybrid system controller executing the methods of the present invention, an ICE, and a MoGen that charges and discharges the battery pack. The MoGen not only provides for propulsion of the vehicle during certain vehicle operating conditions but also replaces an alternator to charge the battery pack in the vehicle and replaces a conventional starter motor to start the ICE. The hybrid drive system of the present invention will utilize the ICE and MoGen to propel or motor the vehicle during the vehicle conditions that are most efficient for the ICE or MoGen operation. The transfer of power between the MoGen and ICE or vice versa is transparent to the operator or driver, as the vehicle will perform as if there is only one drive system propelling the vehicle.
During normal operation of the vehicle when the ICE is running, the MoGen will act as an electrical generator to supply electrical power to the vehicle""s electrical infrastructure (fans, radios, instrumentation, control, etc.) as well as recharging the battery pack. The battery pack and a power transfer device, such as a DC-DC converter, will supply power to the vehicle electrical infrastructure and power the MoGen when it is operating as the motoring device for the vehicle. In the motoring mode, the MoGen is an electrical load drawing current from the battery pack.